The present invention relates to Raman lasers and amplifiers, and more particularly to a tunable Raman laser and amplifier using a plurality of Bragg gratings to define a cascaded resonator configuration.
Fiber Bragg gratings and other grating elements are finding widespread use in both telecommunications and sensing applications. Gratings have also enabled new configurations of fiber lasers and optical signal amplifiers.
Cascaded cavity Raman lasers and optical signal amplifiers, which utilize the stimulated Raman scattering (SRS) in an optical fiber, are known. Raman scattering, a non-linear optical process, is a process in which a small fraction of incident light is scattered by vibrational modes within a non-linear medium (e.g. a silica-based fiber) and is shifted by a known Stokes shift to a typically longer wavelength.
An exemplary cascaded Raman laser 10, as shown in FIG. 1, has an input section 12 including four fiber Bragg gratings 14-17, and an output section 18 also including four fiber Bragg gratings 20-23 that match the gratings in the input section, similar to that described in U.S. Pat. No. 5,323,404. Each pair of matched fiber Bragg gratings (14 and 20, 15 and 21, 16 and 22, 17 and 23) forms an optical cavity, with the gratings having high reflectivity. Pump radiation, having a pump wavelength xcexp, propagates essentially unimpeded through the input section 12 into the optical fiber 24, where most of the radiation will be converted by Raman scattering to radiation at a higher wavelength corresponding to a first order Stokes shift, which is then reflected within the first cavity defined by the first pair of matched fiber Bragg gratings (14 and 20). The reflected radiation of the first resonance cavity is then substantially converted by Raman scattering to a higher wavelength corresponding to the second order Stokes shift, which is reflected within the second cavity defined by the second pair of matched fiber Bragg gratings (15 and 21). The reflected radiation of the second resonance cavity is then substantially converted by Raman scattering to a higher wavelength corresponding to the third order Stokes shift, which is reflected within the third cavity defined by the third pair of matched fiber Bragg gratings (16 and 22). The reflected radiation of the third resonance cavity is then substantially converted by Raman scattering to a higher wavelength corresponding to the fourth order Stokes shift, which is reflected with the fourth cavity defined by the fourth pair of matched fiber Bragg gratings (17 and 23). This radiation at the fourth order Stokes shift is then available for utilization.
The use of fiber Bragg gratings to create a resonant cavity increases the optical power in the fiber and has allowed Raman amplification in shorter lengths of fiber. However, as a result of the relatively narrow bandwidth of the Stokes order, Raman lasers and amplifiers are not tunable over an extended wavelength range.
An object of the present invention is to provide a tunable Raman laser and tunable Raman optical amplifier by which the lasing wavelength of the Raman laser and amplification wavelength of the Raman amplifier can be tuned to allow a more extended adjustment of the output wavelength.
In accordance with an embodiment of the present invention, a tunable Raman laser comprises a pump source that provides pump radiation having a predetermined wavelength. A first optical waveguide includes at least a pair of first reflective elements. Each of the first reflective elements has a respective reflection wavelength. A second optical waveguide includes at least a pair of second reflective elements. Each of the second reflective elements has a respective reflection wavelength, wherein each of the first reflective elements has a respective reflection wavelength substantially the same as a reflection wavelength of a corresponding one of the second reflective elements to form at least a pair of resonant cavities. An optical waveguide, which is optically coupled between the first optical waveguide and the second optical waveguide, provides Raman gain. A tuning device, responsive to a signal representative of a desired output wavelength, stresses the first and second optical waveguides to change the reflective wavelengths of the first and second reflective elements. Portions of each of the first and second optical waveguides have different cross-sectional areas such that when the first and second optical waveguides are stressed, the reflection wavelengths of the first reflective elements change proportionally and the reflection wavelengths of the second reflective elements change proportionally.
In accordance with an embodiment of the present invention, a tunable Raman optical amplifier for amplifying an input light signal having a wavelength is provided. The amplifier comprises a pump source that provides pump radiation having a predetermined wavelength. An optical coupler couples the pump radiation into the input light signal. A first optical waveguide includes at least a pair of first reflective elements. Each of the first reflective elements has a respective reflection wavelength. A second optical waveguide includes at least a pair of second reflective elements. Each of the second reflective elements has a respective reflection wavelength, wherein each of the first reflective elements has a respective reflection wavelength substantially the same as a reflection wavelength of a corresponding one of the second reflective elements to form at least a pair of resonant cavities. An optical waveguide, which is optically coupled between the first optical waveguide and the second optical waveguide, provides Raman gain. A tuning device, responsive to a signal representative of a desired output wavelength, stresses the first and second optical waveguides to change the reflective wavelengths of the first and second reflective elements. Portions of each of the first and second optical waveguides have different cross-sectional areas such that when the first and second optical waveguides are stressed, the reflection wavelengths of the first reflective elements change proportionally and the reflection wavelengths of the second reflective elements change proportionally.